1. Field of the Invention
The present invention relates to a method of generating a motion command for a robot, which is capable of avoiding a influence by natural vibrations when motioning an industrial robot on the basis of a preset motion plan.
2. Related Background Art
If a motion of a robot is repeated at a short pitch, and when a repetitive period approximates the number of natural vibrations of the robot, an arm or the like tends to vibrate, and it is difficult to control it at a high precision. For example, in the case of effecting spot welding of a plurality of portions at a short interval by use of a robot for spot welding, a motion plan for one path goes as shown in FIG. 5A. To be specific, there are repeated an acceleration region till a point of time ta at which a velocity v comes to a maximum velocity, and a deceleration region extending from the point of time ta to a point of time td. According to this motion plan, there is performed a motion at a fixed acceleration .alpha. in the acceleration region, and a decelerating motion at a fixed deceleration -.alpha. in the deceleration region. Variations in the fixed acceleration a and in the fixed deceleration -.alpha. are expressed as shown in FIG. 5B.
The acceleration and deceleration regions shown in FIG. 5B are equivalent to a state where an external force acts on the robot as viewed from a driving side. When the repetitive period of the external force by the acceleration and the deceleration comes to several Hz, this is approximate to the number of natural vibrations of the robot. When the repetitive period of the external force approximates the number of natural vibrations of the robot, it hard to control the motion of the robot, with result that the vibrations are easily produced.
The prior art for restraining residual vibrations based on the natural vibrations of the robot is disclosed in, e.g., JP-A-6-114762. According to this prior art, the residual vibrations when finishing the motion are restrained by measuring a natural vibration period of the robot arm and executing the motion command so that one of an acceleration time and a deceleration time becomes a multiple of an integer of the natural vibration period.
When trying to restrain the natural vibrations from occurring by the method disclosed in JP-A-6-114762, it is required that the number of natural vibrations be at first precisely measured. In, e.g., a jointedl-arm robot, however, the number of natural vibrations changes depending on a position and posture of the robot, and hence there must be complicated processes of measuring the number of natural vibrations corresponding to the variations thereof and carrying out the acceleration and deceleration with a period multiplied by an integer thereof.